These studies are concerned with the definition of the role of phosphatidylinositol turnover in the process of insulin secretion from the rat pancreatic islet. To localize the position of phosphatidylinositol hydrolysis within the stimulus-secretion cascade of insulin secretion, we plan to study the effect of exposure of radioactively labeled islets to various secretagogues, metabolic inhibitors, microtobular stabilizers, ionic concentrations, and membrane stabilizers upon this phenomenon. The time course of incorporation of 2-(3H)-myoinositol into cellular organelles and the effect of carbohydrate stimulation upon its localization will be evaluated by radioautographic techniques. Its pattern of incorporation into the phospholipids of the limiting membranes of several populations of insulin granules (separated by isoelectric focusing) will be studied. The ionogenic properties (surface charge density, calcium binding, and electrophoretic mobility) will be evaluated by microelectrophoresis. The fusion potential of these populations of insulin granules will be studied with the use of islet plasma membranes from the catfish. The protein, phospholipid, and cholesterol content of the insulin granule limiting membrane from catfish islets will be analyzed. Kinetic characteristics, subcellular localization and modulation of the activity of phosphatidylinositol phosphohydrolase in the rat and catfish islets will be investigated. Finally, the effect of experimental diabetes mellitus upon the metabolism of myoinositol and its phospholipid derivatives will be studied in rodents. These studies are designed to provide information concerning the biophysical mechanisms normally responsible for insulin granule migration and for the fusion of its limiting membrane with the plasma membrane of the beta-cell in response to carbohydrate stimulation. Hopefully, these studies may also determine whether abnormalities in the metabolism of myoinositol and its phospholipid derivatives are associated with defective insulin secretion in various forms of experimental diabetes.